1. Field of the lnvention
The invention relates to stepped signal scaling and more particularly to a metal semiconductor field effect transistor (MESFET) designed for operation at frequencies ranging from a fraction of a gigahertz to many gigahertz, the signal transfer (gain and/or attenuation) being stepped in discrete steps over a given range of transfer values. Signal scaling finds major application to antenna arrays.
2. Description of the Prior Art
Monolithic microwave integrated circuit (MMIC) technology has proven useful in electronic circuitry operating at frequencies in the gigahertz range. The technology relies largely on the definition of the active and passive components and their interconnections by a precise, and repeatable photolithographic technique on a monolithic substrate. A preferred substrate material is gallium arsenide. Application of the technology results in a compact and electrically efficient design. The circuits and devices fabricated from this material function well at these frequencies and are capable of precise engineering characterization.
Typically, signal gain in the transmission or reception of signals involving antenna arrays must be adjusted either row by row or element by element. The adjusting means depending upon the number of rows or elements of the array, must be of such accuracy as to preserve the accuracy inherent in focusing or the steering of the array. The adjusting means should be sufficiently broadband as not to distort the signal, often broadband, which is being processed.
It has been proposed that a dual gate MESFET be used for signal gain control. In this application, the signal is applied to the number 1 gate, the gate closest to the source, and a gain control voltage is applied to the number 2 gate, the gate closest to the drain. The control effected by this means is highly non-linear in the conventional device. In addition, because there are many variables effecting the signal transfer value, this approach has lacked precision and repeatability.
An inherent problem in the referenced design is that when the voltage applied to the control gate is changed, both the gain and the phase of the output signal also change. The complex transfer value is additionally dependent upon the biasing, upon the geometry, and upon process dependent characteristics of the device. The result is that the device is difficult to characterize in practice, and when characterized, difficult to employ without compensation. Thus, it has become desirable to find an approach to signal scaling in which the signal gain or attenuation may be adjusted largely independently of phase, and in which the transfer function is precise and repeatable.